The present invention relates generally to radio communications, and more particularly to a method for communicating between multiple stations in a radio communications system using Code Division Multiple Access (CDMA).
Conventional CDMA techniques typically involve the steps of converting information to be transmitted to digital form, coding the digital information with error correction information, and interleaving coded blocks of information to obtain frames or blocks of symbols. Each symbol is then repeated a number of times and the signs of selected symbols are changed according to an orthogonal code pattern. In conventional systems, groups of the same repeated symbol were transmitted adjacent to each other. When propagation comprised multiple paths with relatively delayed propagation times, conventional CDMA signals were received with impaired orthogonality between signals using different orthogonal codes, since one orthogonal code is no longer orthogonal to another code unless they are time-aligned.
In conventional systems, the use of orthogonal Fourier sequences instead of binary orthogonal codes would have been tantamount to transmitting information on different frequencies, i.e., Frequency Division Multiple Access (FDMA). On the other hand, when practicing the invention set forth in the related application, in which single symbols are not repeated adjacent to one another but rather blocks of symbols are block-repeated, the use of Fourier sequences to vary the phase of each block repeat is not equivalent to FDMA and represents a new form of orthogonal coding, the orthogonality of which is less affected by multipath propagation.
Conventional systems often use frequency or timeslot re-use plans to allow transmitters covering adjacent service areas to share frequency spectrum or time without overlapping. For example, in the following U.S. patents, which are assigned to the present assignee and hereby incorporated by reference, both frequency and time re-use patterns and hybrids thereof are described:
The comparable notion of code re-use in CDMA systems has, however, not been implemented commercially. Traditionally, orthogonal codes transmitted from different base stations would be received relatively delayed at a receiver, and therefore no longer orthogonal. Thus, the use of code re-use patterns to control interference levels would have been ineffective. The present invention seeks to overcome the above deficiencies in the art by providing a method which maintains orthogonality to thereby allow for the use of code re-use patterns.
In an exemplary embodiment of the present invention, a coding method comprises the step of converting information to be transmitted into digital form through the use of, for example, an analog-to-digital converter. The digital information is then error-correction coded using, for example, convolutional coding, block-coding or Reed-Solomon coding in order to improve error tolerance. The coded symbols are then assembled into frames containing N symbols for transmission.
Each coded information symbol within a frame is then repeated a first number of times L1 in succession and the sign of select repeated symbols, determined by a spreading code generator, is changed to produce a symbol block of Nxc3x97L1 symbols. The symbol block is then repeated a second number of times L2, wherein for each block repeat, a block sign change is applied to all symbols of the same block; the sign for each block may also be supplied by the spreading code generator. The resulting L2xc3x97Nxc3x97L1 coded and repeated symbols are then modulated upon a radio frequency carrier and transmitted to a receiver simultaneously with similar symbols intended for other receivers, whereby different information is transmitted to a plurality of receivers. Signals coded for simultaneous transmission to different receivers may comprise a greater number of information symbols repeated a reduced number L1 times or a lesser number of information symbols repeated a greater number L1 times, while still maintaining the same number of repeated symbols in a block.
A receiver for decoding information according to the present invention comprises means for receiving a composite radio signal bearing information for a plurality of receivers and means for converting the composite radio signal to a stream of representative numerical samples and storing the samples in memory during at least one information transmission frame. The stored samples are then compressed in number by a factor L2 by combining corresponding samples from each of the L2 block repeats, using additive or subtractive combinations according to the signs supplied by a local spreading code generator. As a result, the wanted signal components of the samples combine constructively while a high proportion of unwanted signal components cancel.
The compressed samples are then further compressed by a factor L1 by combining samples within the compressed block that correspond to repeated symbols and using another sign pattern from the local spreading code generator to affect additive or subtractive combining such that wanted signal components are enhanced relative to unwanted signal components. The signal samples, now doubly compressed by a factor L1xc3x97L2, are then error-correction decoded using, for example, a convolutional decoder or Reed-Solomon decoder in order to reproduce the transmitted digital information symbols.
The information symbols may then be digital-to-analog converted, if necessary, to reproduce an analog information signal, such as a voice signal.
In one preferred embodiment, the number of block repeats L2 is two. A first group of information signals to be transmitted from a transmitter to respective receivers located within a first service area is coded using block repeat signs ++ while a second group of information signals is coded using block repeat signs +xe2x88x92. The first group of information signals are, for example, intended for receivers situated at greater distances from the transmitter than the receivers for the second group of information signals.
A second transmitter for transmitting signals to receivers in a second service area, bordering or partially overlapping the first coverage area, codes information for transmission similarly, but the block repeat signs may be reversed (i.e., using sign pattern ++ for transmitting to the more nearby receivers and +xe2x88x92 for transmitting to the more distant receivers). In this way, signals, which are transmitted at high power and therefore intended for distant receivers in one service area, suffer reduced interference from signals transmitted at high power in an adjacent service area. Also, signals transmitted at low power to nearby receivers suffer less interference from strong signals transmitted in the same coverage area to distant receivers.
In a second embodiment of the invention, the L2 block sign changes are replaced by L2 block phase changes. The L2 repeated blocks are transmitted with a phase rotation for each block of 0, Phi, 2 Phi, 3 Phi, . . . , L2 Phi where Phi is a block phase increment of zero or an integral multiple of 2Pi/L2.
The receiver for the phase-rotated repeat blocks combines L2 corresponding samples from each of the repeated blocks by first derotating the phase of a sample by its known block phase rotation to align the L2 samples in phase before adding them.
In a third embodiment, L2 is equal to 3 and Phi is zero degrees for a first group of signals, 120 degrees for a second group of signals, and 240 degrees for a third group of signals. The service area covered by a first transmitter is divided into sectors. The first group of signals are transmitted to the most distant receivers, the second group are transmitted to receivers at an intermediate distance, and the third group of signals are transmitted to the nearest receivers.
The assignment of a block repeat phase according to receiver distance is then permuted in adjacent service areas covered by other transmitters to form a 3-cell code re-use pattern, analogous to a frequency re-use pattern used in conventional cellular radio telephone systems.